The Castle Creek valley, south of Aspen, Colorado is host to several great trails. One I'm eager to do again is Cathedral Lake.
The trail is moderately steep and rocky, but not difficult, and hikers are rewarded with great views on the way up and a spectacular scene at the top. The above shot was taken in late summer, when the water levels were lower. I'm eager to do this hike earlier in the season, along with its neighbor, American Lake.
The trailhead for Cathedral Lake is not too far past the ghost town of Ashcroft, on the right hand (south) side of the road.